Perception of the second of two rapidly sequential targets is impaired if the temporal lag between them is short (“attentional blink”). We show how this can arise from an exogenously-triggered change in task-set. Observers saw a rapid stream of distractors (digits) centred on the screen. In the THREE-LETTER condition, observers reported three successive letters embedded in the stream of digits. In the TWO-LETTER condition, a digit replaced the second letter, with observers reporting only the two letters. Identification accuracy for the final letter was significantly higher in the Three-letter than in the Two-Letter condition, even though memory load was higher in the former. We propose (1) that the intervening digit triggered an involuntary change in task-set from letters to digits, and (2) that this was only possible while the system was busy identifying the first target. Thus, perception of the final letter in the Two-Letter condition was impaired because it did not fit the new task-set imposed by the intervening digit.

Additional studies confirmed this account. First, the final-letter deficit remained when observers reported the intervening digit as well as the two letters. Second, the deficit vanished when the intervening distractor in the two-letter condition was rendered virtually invisible by an abrupt spatial shift in the location of the stream after the first target. Third, a central stream of preceding digits interfered much less with target-letter identification than a stream of preceding letters.

We propose that an endogenously-established task-set can be maintained quite effectively during the period in which a target letter is anticipated, permitting the efficient exclusion of task-irrelevant distractor digits. However, as soon identification of the target letter begins, the task set becomes vulnerable to alteration by exogenous distractors, making it now optimally tuned to the stimuli exemplified by the distractor digits instead of the letters.